This invention relates to programmable integrated circuits, such as programmable logic devices (PLDs), and, more particularly, to the implementation of filters in specialized processing blocks which may be included in such devices.
As applications for which PLDs are used increase in complexity, it has become more common to design PLDs to include specialized processing blocks in addition to blocks of generic programmable logic resources. Such specialized processing blocks may include a concentration of circuitry on a PLD that has been partly or fully hardwired to perform one or more specific tasks, such as a logical or a mathematical operation. A specialized processing block may also contain one or more specialized structures, such as an array of configurable memory elements. Examples of structures that are commonly implemented in such specialized processing blocks include: multipliers, arithmetic logic units (ALUs), barrel-shifters, various memory elements (such as FIFO/LIFO/SIPO/RAM/ROM/CAM blocks and register files), AND/NAND/OR/NOR arrays, etc., or combinations thereof.
One particularly useful type of specialized processing block that has been provided on PLDs is a digital signal processing (DSP) block, which may be used to process, e.g., audio signals. Such blocks are frequently also referred to as multiply-accumulate (“MAC”) blocks, because they include structures to perform multiplication operations, and sums and/or accumulations of multiplication operations.
For example, STRATIX® II and STRATIX® III PLDs sold by Altera Corporation, of San Jose, Calif., include DSP blocks, each of which includes a plurality of multipliers. Each of those DSP blocks also includes adders and registers, as well as programmable connectors (e.g., multiplexers) that allow the various components to be configured in different ways. In each such block, the multipliers can be configured not only as individual multipliers, but also as a number of smaller multipliers, or as one larger multiplier. In addition, one complex multiplication (which decomposes into two multiplication operations for each of the real and imaginary parts) can be performed.
Such a DSP block may be configured as a finite impulse response (FIR) filter. Each block may be used to perform the summation of a plurality of multiplications to form a sub-block of a longer FIR filter.
Many types of FIR filters may be encountered. Two of those types are an interpolation FIR filter—in which the number of samples is increased by a factor of n by inserting (“interpolating”) n−1 samples between adjacent samples—and a decimation FIR filter—in which the number of samples is decreased by a factor of n by removing n−1 out of every n samples. A DSP block that may be configured as different types of filters, including an interpolation FIR filter and a decimation FIR filter, is shown in copending, commonly-assigned U.S. patent application Ser. No. 11/447,370, filed Jun. 5, 2006, which is hereby incorporated by reference herein in its entirety.
As described in the above-incorporated application, when a programmable logic device including a DSP block is configured as a decimation filter, portions of the DSP block must run at several times (e.g., three or four times) the input data rate, because the filter must operate several times with different coefficients on the same data before the data is clocked out of the data registers. This is referred to as “overclocking.”
It would be desirable to be able to provide a DSP block in a programmable logic device that could perform decimation filtering without having to overclock the DSP block, especially when processing high input data rates.